I see 2 in your photo, but it's not really an emergency. They aren't going to wipe out your tank and can be treated at your leisure. Flatworm exit is a good start, you can certainly try a fish to control them as well but that is far less likely of being successful. The only real danger is killing 1000s of them at once, as they release a toxin when they die. During their day to day life they are no danger and actually may eat algae ect.
Additionally if you don't have an infestation there is always the chance you will never have one, while they look like the red ones that are very invasive it's tough to tell from your photo. There are certainly times where people just get a few and that's it.